Simply put, what is a U-Value?

Without getting too technical, lets explain U-Values!

A U-Value is a measurement of how effective a material is as an insulator. Which is extremely important for Windows and Skylights, seeing as they are going to be part of your home.

Basically, when we install any Window or Skylight we want to make sure that we are not losing heat through this part of the house. Losing heat will cost us more money and use more energy, especially in the winter time.

The lower the U-Value figure, the better! Let’s explain why…

How is U-Value measured?

Calculating a U-Value is actually quite simple. The U-value can be calculated by finding the reciprocal of the sum of the thermal resistances of each material making up the building element in question. Note that, as well as the material resistances, the internal and external faces also have resistances, which must be added. These are fixed values.

U-Values are measured in watts per square metre per Kelvin or shortened to (W/m²K).

Nearly all manufacturers of Roof Windows and Rooflights will be able to give you U-Values of their products. One way a manufacture will test their product is in a “Hot Box”. This is a machine that can test how well an insulator their product is by exposing the product to hot and cold elements, like it would in your home. (Inside your home is warm, outside is cold.)

The image on the right here is an example of how this works.

So, lets take an example! Let’s say that a Double Skin Polycarbonate Roof Dome has a U-Value of 2.8W/m²K… for every degree difference in temperature between the inside and outside of the glazing, 2.8 watts will be transmitted every square metre.

But lets also take a Glass Rooflight. The Glazing in this kind of Rooflight may have a Gas Filled Cavity, usually Argon, which gives better insulation. So, let’s say for example the U-Value is 1.1W/m²K. For every degree difference in temperature between the inside and outside of the glazing, 1.1 watts will be transmitted every square metre. So less heat loss.

Lower the U-Value, less heat is transmitted, or lost, through the glazing.

Making Sense?

U-Value Explained

U-Values and Building Regulations

Getting a good U-Value for your Rooflight is not only important for making sure your home is well insulated, but it’s worth noting that Building Regulations will require a minimum U-Value for different parts of a new build. This will be anything from External Walls, Floors, Roofs, Glazing (Windows and Rooflights) and Doors.

This is covered in Part L (Conservation of fuel and power) of the building regaulations, which is listed on our website.

It is worth noting that the specification for the notional domestic building which is referred to in Part L1A, has considerably lower values.

For example:

External wall: 0.18 W/m²K.
Floor: 0.13 W/m²K.
Roofs: 0.13 W/m²K.
Windows, roof windows, glazed rooflights and glazed doors: 1.4 W/m²K.

Although Building Regulations has limits for U-Values, the overall thermal performance of buildings is now assessed is a much more complex procedure.

For example, non-domestic buildings, the Simplified Building Energy Model (SBEM) developed by the BRE for the Department for Communities and Local Government, determines the energy performance of a proposed building by comparing its annual energy use with that of a comparable notional building.

And for dwellings, energy performance is measured using the Government’s Standard Assessment Procedure which is also know as (SAP).

What is R-Value?

R-Value, which shouldn’t get confused with U-Value, is a measurement of thermal resistance rather than thermal transmission, are often described as being the reciprocal of U-values.

R-Value – Thermal Resistance
U-Value – Thermal Transmission

Note: R-Value Does not include surface heat transfers.

What is G-Value?

G-Value is another important figure that you should be looking at when it comes to purchasing a new Rooflight for your home.

G-Value is the solar transmittance through translucent and transparent materials such as glass and is very important for determining the solar heat gain into the space they enclose during sunny conditions.

Solar heat gain can be beneficial in the winter, as it reduces the need for heating, but in the summer can cause overheating.

Ever stood inside a greenhouse in the summer? Or inside a Polycarbonate conservatory that everyone had installed in the 90’s? This is a classic example of G-Value, high heat buildup, lots of solar heat gain.

If you are planning to install large areas of glazing into your home (Large Rooflights), you will certainly want to get in touch with us to ensure we offer a product that will balance the solar heat gain.

G-Value – Solar Heat Gain

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Frequently Asked Questions

What size is the ideal skylight?

The optimal size of a skylight depends on several factors. Generally speaking, skylights should be installed to cover about 10-15 percent of your room’s floor area. For a room measuring approximately 400 square feet, a skylight should be 4 feet by 8 foot. Be aware that bigger rooms might require several smaller skylights, rather than one large.

If you are looking for a skylight that will reduce glare or protect furniture from the sun, make sure you check out which models have filtering systems. For seasons where temperatures fluctuate significantly, it is worth looking for designs that feature low-e glazing or tinted windows to maintain optimal climate control. The process of choosing the right size or type of glass can be complicated, but it’s worth it if it allows you to lighten your home and change its appearance.

What are the most common problems with skylights

Skylights are an excellent way to add natural light to your home, but there are some issues.

Moisture is an issue. As moist air can easily enter the skylight and condense onto the frame and glass, this can cause mold or rot. Failing seals can also cause air leakage and raise energy bills.

Problems can also be caused by improper installation. A poorly sealed skylight could not provide enough insulation against extreme temperatures. This could result in a higher heating bill in winter or excessive heat during summer months.

Uneven glazing is another possible problem with skylights. This happens when the glass is not fitted snugly into the window openings or if the sealant around the edges fails. Untreated mismatches can lead to water intrusion and further problems with moisture control.

Finally, improper ventilation may be an issue as well as dust accumulation from inadequate air circulation from outside into your home; it’s important to ensure ventilation fans are included for the best protection against dirt buildup and sweltering temperatures in summer months.

Are skylights better than windows for letting in light?

The answer is generally yes, skylights can let in more light than windows due to their positioning. Skylights allow for more natural light to be reflected from the sky, and directed into the living areas below. The angle at which they are installed lets them shed more sunlight on any given day than traditional windows can, often leading to brighter rooms with an enhanced atmosphere.

Skylights also have unique features that let you bring in light even in the darkest hours. Skylights allow natural sunlight to pass through the roof and create an indoor environment that is warm and welcoming for all year comfort, particularly in areas where there is less sun.

Modern window designs feature innovative technology that allows them to capture the same amount daylight as a skylight but also provide privacy and protection when closed. However, installing multiple traditional windows still won’t provide the same level of illumination as one room-sized skylight in most cases.

Before you decide on which solution is best for your home or workplace, consider your needs and preferences.

Should you buy curb-mounted, deck-mounted, or both?

This will depend on your roof’s limitations. You can mount curb-mounted or deck-mounted skylights directly on the frame of your roof deck.

A deck-mounted skylight is a good option if you don’t have enough space to place a curb around your house’s exterior. In this case, you’ll use flashing that is compatible to your roof’s material and fasten it to your roof frame or decking.

Curb-mounted skylights, however, offer greater flexibility for homeowners looking for a wider variety of sizes and shapes available for their installation. You also have more choice in styles, such as sun tunnels or ventilated ones. This is useful if you want the maximum amount of natural light to enter your interior space. It is important to have enough space between the windows and the wall to mount them correctly.

Both can be used, depending on individual needs. In order to decide which type is best suited for your specific situation, you should measure the available space and consider any restrictions or framing plans.


  • The 26% tax credit is available on solar skylights AND accessories, even if the solar accessory is purchased with a non-solar skylight. (
  • If the room has numerous windows, the skylight should not be more than 5 percent of the floor area and not more than 15 percent in rooms with few windows. (
  • Skylights let 30% more natural light in as compared to conventional wall windows. (
  • If you replace your water heater the following year, you would be eligible for another 30% tax credit, up to $2,000 plus up to $600 if you need an electric panel upgrade to accommodate the new water heater. (
  • The flat roof skylight can help reduce this noise by up to 50%, making it much easier to relax when inside your home. (

External Links

How To

How to Install a Rooflight Yourself

You can install a skylight by yourself. It’s easy to do and enjoyable DIY project. The process requires precise measurements and the ability to work on your roof. It is important to understand what you are getting into. Experts are recommended if you feel overwhelmed by the scope of the job.

Before you begin the installation process, make sure you measure the space you wish to place your skylight in. Also, ensure that you are familiar with your roof’s structure and weatherproofing requirements. Once you are certain that your roof is capable, select a style to fit the job and buy the necessary materials.

Use shingles for temporary cover and caulk long-term weatherproofing around the skylight hole base to ensure a perfect installation. To seal any gaps or reduce heat loss, metal flashing can be used between layers of shingles to cover the skylight hole.

Final, make sure to place the new window in the construction opening. This step will help you to properly position your window. However, foam frame caulk can also improve performance by allowing for space between the construction opening and the panel frame.

Once everything is properly assembled, be sure to remove protective tape from the window film. Then, engage latch hooks to form a permanent seal against outside elements. Finally, we will move onto exterior caulking using polyurethane foam or silicone bathtub from a hardware store depending on our budget and performance preferences.

Did you miss our previous article…